The Ultimate Guide to Bonsai Repotting

To become proficient as a Bonsai artist, it’s essential to perfect your repotting technique. Mastering this skill will significantly improve the development and quality of your Bonsai cultivation.

Bonsai repotting is an opportunity to reduce root growth, provide the plant with a fresh growing medium, and select a new pot for your Bonsai. Repotting also allows one to address any root rot that may have developed and to evaluate your previously used medium.

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Introduction to Bonsai Repotting

The most crucial and fundamental technique in cultivating bonsai trees is probably repotting. Sadly, it is also the worst done and most misinterpreted.

Repotting Bonsai Trees
Repotting Bonsai Trees

Your experience with bonsai trees will change once you learn why we must repot and how to do the job effectively. 

Everything about Bonsai becomes easier once you comprehend the fundamental biochemistry underlying the growth pattern of trees and plants.

Why You Need to Repot Your Bonsai

Bonsai grow in shallow pots with a limited soil volume. It takes about two years for the roots to fill the available space and to start strangling each other, causing the plant to suffer.

Repotting is essential to maintain healthy growing conditions and may (or may not) include root pruning. Repotting does not stunt the tree; on the contrary, it benefits it.

A healthy tree that has been properly replanted will develop considerably more than it did while it was in its previous root-bound pot.

Roots Grow Affects Plant Health

For plants to live, they need a supply of 20 bioavailable chemical compounds and light. The most essential for photosynthesis are water, carbon dioxide, and sunlight, which produce food for cellular growth (sugars and starches).

Respiration uses oxygen and creates, as by-products, carbon dioxide and water. The roots need oxygen; typically, the soil should have more than five percent air to avoid anaerobic conditions.

Repotting ensures this air supply as the growing roots compact the soil, limiting the available space for oxygen.

Have a look at my article on Bonsai soils to get a better understanding of the soil’s essential physical characteristics:

  • The soil mix density or specific gravity (SG). Water is the standard with an SG of 1 (ice is 0.9, and therefore floats on water, less dense). Specific gravity is a factor of weight to volume. Thus, the density of your soil mix is its dry weight divided by its volume.
  • The Saturation Porosity (SP) is the difference between the fully saturated and drained mix as a factor of its volume. The SP (the fractional air content) is (5-4.5)/5 = 0.1 or 10%. Typical farming soil ranges between 2 to 7%. An SP below 5% indicates anaerobic conditions where roots cannot breathe.
  • The Field Capacity (FC) is the difference between the drained wet weight and the dry weight of the mix divided by the volume.

Because a bonsai grows in a relatively small amount of soil, it can quickly fill its soil pores with roots and drastically reduce the saturation porosity of the soil.

The reduced oxygen available to the roots stifles their growth, slows the growth, and less growth means a reduction in food production through photosynthesis.

Repotting is necessary to stop the condition before it worsens to a point where the health of the trees is compromised. Recognizing a tree’s warning signs that repotting will soon be required takes skill.

When to Repot Your Bonsai

Reduction of Bonsai Root System in Compacted Soil
Reduction of Bonsai Root System in Compacted Soil

Once the soil becomes quite dense and hard, repotting should be done. This does not imply that the soil in a bonsai tree must always be loose and granular; on the contrary, there may be reasons to retain a tree in a slightly pot-bound state for an extended time.

This can help the bonsai develop and limit the growth, resulting in shorter internodes, more flowers, and a more worn-in appearance.

However, some action will be needed once it becomes challenging to get enough water into the soil.

Never repot bonsai by a calendar! Always be guided by the growth patterns of the tree. Here are some signs to look out for:

  • Difficulty wetting the tree’s soil.
  • A slowing rate of growth over previous seasons.
  • Reduced uptake of water in summer.
  • Rapidly reducing leaf size.
  • Slight yellowing of foliage color.
  • Early leaf drop in autumn.
  • Reduced gloss on the foliage.
  • Liverwort and algal slime form on the surface of the soil.
  • Dieback of fine twigging in winter.
  • The root ball gradually rises in the pot.
  • Reduced leaf viability (discoloring and dropping after a few weeks in summer).

Steps for Repotting Your Bonsai

Removing a Bonsai Tree from Its Pot

A bonsai must be removed from its pot before being placed in a new one, which requires removing all cables that protrude through the pot’s base.

After removing the tie-down wires, I often remove the drainage screen clamps.

Create a shallow trough between the rootball and the pot’s sides using a small sickle or sod knife. The process’s riskiest step is this one. One hand keeps the pot steady while the other quickly manipulates a sharp object. Be careful with your fingertips!

Work on the two short sides and one large side of the square, rectangular, and oval pots. After roughly half of the diameter has been cleaned, trees typically come out of circular pots.

Always scrape to the bottom of the saucepan. Stopping short of this bulk can prevent the tree from emerging since roots tend to circle at the bottom.

Prepare Your Bonsai Pot

Prepare the Pot for Bonsai
Prepare the Pot for Bonsai

The pot where the bonsai will be planted must be prepared next. I can’t stress how important it is not to scrub the pot too vigorously; instead, remove the dirt while protecting the patina.

Measure the length of the wire roughly four times the drainage hole’s diameter to cover the holes. With pliers, complete the “z” bends you started with your hands on the wire.

The ends should then be curved until they barely fit into the drainage holes. A secure fit will stop the screen from swiveling, keeping pests out and soil in.

Measure the tie-down wire once the screen has been installed. Measure the pot’s two long sides and one short side to determine the proper length of the tie-down wire for rectangular and oval-shaped pots.

Adjustments are necessary due to different forms and drainage hole configurations. With practice, getting the length right becomes simpler.

Prune the Roots

Pruning The Bonsai Roots
Pruning The Bonsai Roots

Ever-expanding roots are addressed through root trimming. A tree grows new white feeder roots from the cut ends of the old lignified roots after being correctly pruned and placed back in the soil, making the tree much stronger.

A smaller, more active root system will get more nutrients from the soil than a larger, less productive one. Feeder roots always spread outward, and getting farther away from the tree is one feature of root growth that will eventually slow a tree’s growth.

In bonsai cultivation, once the root tips hit the inside of the pot, they grow in a circular motion and get progressively further from the tree itself.

I have seen 12 feet of root uncoiled from a 10-inch-wide bonsai pot. Moving water and nutrients that far; much better, a couple of inches!

Securing the Tree in the Pot

Securing the Bonsai Tree To The Pot
Securing the Bonsai Tree To The Pot

It’s time to assemble the tree and container after they are both ready. The pot first receives a pumice drainage layer.

I may omit this step if the pot were much thinner or the climate warmer, but I’ve discovered that my trident maples thrive when a pumice drainage layer is used.

After spreading out the drainage layer, I add my deciduous bonsai mix, which consists primarily of akadama with pumice, lava, and charcoal.

When I pour the mixture into the pot, I usually make a mound in the middle to help prevent air pockets from emerging when the tree is set, and this is significant if the rootball’s base is concave.

The next step in the procedure is one of the most crucial: setting the tree. Although it is not a difficult maneuver, attention must be taken to ensure the tree is placed properly.

Check the tree’s position, the rootball’s height, the tree’s tilt, and the front once you’ve positioned it. Before wiring the tree into place, make any changes that are required.

Frequently Asked Questions

When should you repot Bonsai?

You should repot bonsai during early spring. As soon as the buds swell, you know that sap is rising, but the tree is still dormant. This is the perfect time for bonsai repotting.

Do you need special soil for Bonsai?

Special bonsai soil is required for these trees. Store-bought or homemade soils can utilize Pumice, Akadama, Red Lava Rock, Organic Matter, Gravel, Worm Castings, and bark chips. Normal garden soil is not suitable for bonsai.

Should bonsai roots be exposed?

Bonsai roots, or Nebari, can be exposed. It is desirable to see the Nebari when growing bonsai trees. Uncovering the roots this way does not cause any issues for the bonsai tree. In nature, trees expose their roots all the time.

In Closing

One of my favorite Bonsai techniques is root pruning. You only see how your root system grows when repotting or root trimming. Instead of being afraid, approach root pruning, follow my advice, and you’ll have strong roots and a flourishing bonsai.

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